Fixing device with meandering prevention members and image forming apparatus including the same

Abstract

A fixing device employing resistance heating system includes: a fixing roller being loosely inserted into an endless fixing belt; a pressure member being in pressure-contact with the fixing belt to form fixing nip; meandering prevention members preventing the fixing belt from meandering; prevention member holders holding the meandering prevention members for rotation independently from the fixing roller. Each meandering prevention member has rotation center inside circle, where, when seen in rotation axis direction of the fixing roller, the circle has center coincident with midpoint between two focal points of ellipse approximating belt rotation path of the fixing belt, and has radius equal to distance from the center of the circle to straight line passing through rotation center of the fixing roller and center of the nip in rotational direction of the fixing roller.

What is claimed is: 
     
       1. A fixing device comprising:
 an endless fixing belt that includes a resistance heating layer that generates Joule heat when electric power is fed thereto and a pair of electrode parts that feed electric power to the resistance heating layer; 
 a pair of power feeding members that are each in abutment with an outer circumferential surface of a corresponding one of the electrode parts to feed electric power to the resistance heating layer through the electrode part; 
 a fixing roller that is loosely inserted into the fixing belt; 
 a pressure member that is in pressure-contact with an outer circumferential surface of the fixing belt to form a fixing nip; 
 a pair of meandering prevention members that are each provided facing a corresponding one of sides of the fixing belt in a width direction of the fixing belt, and prevent the fixing belt from meandering in the width direction; and 
 a pair of prevention member holders that each hold a corresponding one of the meandering prevention members, such that the meandering prevention member rotates independently from the fixing roller, wherein 
 the meandering prevention members are each held so as to have a rotation center positioned inside a circle, 
 where, when seen in a rotation axis direction of the fixing roller, the circle has a center coincident with a midpoint between two focal points of an ellipse approximating a belt rotation path of the fixing belt, and has a radius equal to a distance from the center of the circle to a straight line passing through a rotation center of the fixing roller and a center of the fixing nip in a rotational direction of the fixing roller. 
 
     
     
       2. The fixing device of  claim 1 , wherein
 the meandering prevention members are each held so as to have the rotation center that is coincident with the center of the circle. 
 
     
     
       3. The fixing device of  claim 2 , wherein
 when seen in the rotation axis direction of the fixing roller, 
 the ellipse approximating the belt rotation path of the fixing belt is included in a circle enclosing the belt rotation path, and includes therein a circle within the belt rotation path. 
 
     
     
       4. The fixing device of  claim 1 , wherein
 the meandering prevention members are each held, such that, in a diameter direction thereof in a plane perpendicular to a rotation axis thereof, the rotation axis is positioned equally distant from the nip and from an outermost circumference thereof that is in abutment with the fixing belt. 
 
     
     
       5. The fixing device of  claim 1 , further comprising
 a housing that houses therein the fixing belt, the power feeding members, the fixing roller, the pressure member, the meandering prevention members, and the prevention member holders, wherein 
 the prevention member holders are fastened to an inner wall of the housing, and 
 the prevention member holders rotatably hold the fixing roller via a bearing, and each rotatably hold the corresponding meandering prevention member via a bearing. 
 
     
     
       6. The fixing device of  claim 1 , wherein
 the meandering prevention members each have an outer circular circumference, and 
 the prevention member holders each hold the corresponding meandering prevention member by bringing three or more rollers into abutment with the outer circular circumference of the meandering prevention member. 
 
     
     
       7. The fixing device of  claim 1 , wherein
 the meandering prevention members are in sliding contact with the fixing belt so as to be driven by the fixing belt to rotate. 
 
     
     
       8. The fixing device of  claim 1 , further comprising
 a driving unit that drives the meandering prevention members to rotate in accordance with rotation of the fixing belt. 
 
     
     
       9. The fixing device of  claim 8 , wherein
 the driving unit includes: 
 a detection subunit that detects a rotational speed of the fixing belt; and 
 a speed adjustment subunit that adjusts a rotational speed of the meandering prevention members in accordance with the rotational speed of the fixing belt detected by the detection subunit. 
 
     
     
       10. The fixing device of  claim 1 , wherein
 the power feeding members are each provided, such that, when seen in the rotation axis direction of the fixing roller, the power feeding member is positioned in a region that is immediately upstream of the fixing nip among four regions partitioned by a first straight line and a second straight line, 
 where the first straight line passes through a rotation center of the fixing roller and the center of the fixing nip in the rotational direction of the fixing roller, and the second straight line passes through the rotation center of the fixing roller and is perpendicular to the first straight line. 
 
     
     
       11. An image forming apparatus comprising a fixing device,
 the fixing device comprising: 
 an endless fixing belt that includes a resistance heating layer that generates Joule heat when electric power is fed thereto and a pair of electrode parts that feed electric power to the resistance heating layer; 
 a pair of power feeding members that are each in abutment with an outer circumferential surface of a corresponding one of the electrode parts to feed electric power to the resistance heating layer through the electrode part; 
 a fixing roller that is loosely inserted into the fixing belt; 
 a pressure member that is in pressure-contact with an outer circumferential surface of the fixing belt to form a fixing nip; 
 a pair of meandering prevention members that are each provided facing a corresponding one of sides of the fixing belt in a width direction of the fixing belt, and prevent the fixing belt from meandering in the width direction; and 
 a pair of prevention member holders that each hold a corresponding one of the meandering prevention members, such that the meandering prevention member rotates independently from the fixing roller, wherein 
 the meandering prevention members are each held so as to have a rotation center positioned inside a circle, 
 where, when seen in a rotation axis direction of the fixing roller, the circle has a center coincident with a midpoint between two focal points of an ellipse approximating a belt rotation path of the fixing belt, and has a radius equal to a distance from the center of the circle to a straight line passing through a rotation center of the fixing roller and a center of the fixing nip in a rotational direction of the fixing roller.